Dang... I am not one to hate on motor flushes, it's on a case by case basis.... Usually, we will put a can of Gunk flush before a normal OCI... once every few years. It's not a huge nasty undertaking, and again, the motor I'm dealing with are usually not filth ridden..
Not so long ago I had the valve cover off of my work car when I put a timing belt on... it was absolutely spotless. I was really amazed at how clean it was.
I will say, that at a normal OCI for us, the oil coming out is a very dark amber, but light can be seen though the stream. Pour in a little flush, and that dump out definately took some more dirt with it.
With any mishandled motor, it's always... try, and hope for the best.
There's always two crowds here... the "Engine Cleaning Is Part of Maintenence"... and "Engine Cleaning Is Evil".....